The present invention relates to a method for producing a powder compact and a method for manufacturing a magnet, and also relates to a powder press used for compaction of powder and a method for driving the powder press. The present invention particularly relates to a compaction technique especially suitable for production of a compact having a shape in which the size measured in the pressing direction (direction in which uniaxial pressure is applied) is greater than the size in the direction perpendicular to the pressing direction (for example, a rod shape and a cylinder shape).
In the field of powder metallurgy, various methods have been employed for imparting a shape to powder. Among them, in particular, in the field of manufacture of sintered magnets, widely used is a method for compacting magnetic alloy powder (magnet powder) with a powder press.
A conventional method for producing a green compact of magnetic alloy powder will be described with reference to relevant drawings.
FIGS. 1A through 1C are cross-sectional views schematically illustrating the operation of a powder press of a withdrawal type. The press includes: a die 2 having a through hole for formation of a cavity 1: and an upper punch 3 and a lower punch 4 for compacting powder in the through hole. The press further includes an upper ram 5 and a lower ram 6 coupled to driving devices not shown. In the illustrated conventional example, the upper ram 5 is driven upward and downward together with the upper punch 3, while the lower ram 6 is driven upward and downward together with the die 2. The lower punch 4 is kept at a fixed position with respect to a main body 10 of the press.
Using the press having the above construction, a compact is conventionally produced in the following manner.
As shown in FIG. 1A, the top end portion of the lower punch 4 is located inside the through hole of the die 2 to define the cavity 1. The cavity 1 is filled with material powder. Next, as shown in FIG. 1B, the upper punch 3 is lowered to allow an end portion thereof to be inserted in the through hole of the die 2, so that the powder is compacted between the upper punch 3 and the lower punch 4 (uniaxial compaction). Thus, a green compact 7 of the filled powder is produced. Thereafter, as shown in FIG. 1C, a step of ejecting the compact 7 from the die 2 (xe2x80x9cdraw-out stepxe2x80x9d or xe2x80x9cpush-out stepxe2x80x9d) is performed. In the illustrated conventional example, the die 2 is lowered while the lower punch 4 and the compact 7 are kept unmoved, and the upper punch 3 is lifted.
The above operation will be described in detail with reference to FIGS. 2A and 2B.
In FIG. 2A, lines A and B represent positions of the upper punch 3 and the die 2, respectively, and line C represents pressure P applied to the top end face of the compact 7. The compact 7 receives pressure, not only from the upper punch 3, but also from the lower punch 4 and the die 2. Herein, however, for convenience, only the pressure applied from the upper punch 3 to the compact 7 is specifically called xe2x80x9ccompact pressurexe2x80x9d, and the magnitude of the compact pressure is denoted by xe2x80x9cPxe2x80x9d. The pressure P in FIG. 2A refers to this xe2x80x9ccompact pressurexe2x80x9d.
xe2x80x9cS1xe2x80x9d, xe2x80x9cS2xe2x80x9d, xe2x80x9cS3xe2x80x9d, and xe2x80x9cS4xe2x80x9d in FIGS. 2A and 2B respectively represent the step of compacting powder, the step of lifting the upper punch 3 at a minimal speed, the step of ejecting the compact 7, and the step of lifting the upper punch 3 at a high speed. These steps will be described in order as follows.
In the step S1, the filled powder is compacted by applying a large pressure PC to the powder to form the compact 7. The compaction in a narrow definition is completed in this step. The compact 7 is in the state of being pressed inside the die 2. At time t1, the step S2 is started where the upper punch 3 is lifted gradually at a minimal speed. With this gradual lift of the upper punch 3, the compact 7 in the pressed state expands as an elastic body in the direction opposite to the pressing direction. Once the compact pressure P reaches PH ( greater than 0), the minimal-speed lift of the upper punch 3 is halted.
At time t2, the step S3 of ejecting the compact 7 is started. During this step, the compact 7 is held between the upper punch 3 and the lower punch 4, and the pressure PH of substantially a constant value is kept applied to the compact 7 from the upper and lower punches 3 and 4.
At time t3, at which the compact 7 has been completely ejected from the die 2, the step S4 is started where the upper punch 3 is lifted at a high speed. With this highspeed lift of the upper punch 3, the compact pressure P abruptly drops, and becomes zero when the upper punch 3 is detached from the top end face of the compact 7.
The above compacting method is called a hold-down method (see xe2x80x9cPowder Compaction and Processingxe2x80x94From Powder to Nearnet Shapexe2x80x9d ed. by Japan Society for Technology of Plasticity and Japanese Laid-Open Patent Publication No. 6-81006), which has a feature that the compact 7 is ejected from the die 2 while a constant holding pressure (PH) is applied to the compact 7 from the upper punch 3. This method can prevent xe2x80x9cdetaching fracturexe2x80x9d of the compact 7, a phenomenon that may be generated in the course of ejecting the compact 7 from the die 2.
Hereinafter, the mechanism of generation of detaching fracture will be described with reference to FIGS. 3A and 3B.
FIG. 3A schematically illustrates the state where the die 2 that has just started moving downward applies friction to the periphery of the compact 7. FIG. 3B schematically illustrates the state where the top end portion of the compact 7 is exposed outside as the lowering of the die 2 proceeds. Since the pressed compact 7 is an elastic body, it tends to expand in the direction shown by arrow Q1 as a pressure P1 applied to the compact 7 from the upper punch 3 decreases (springback phenomenon). Once the pressure P1 is removed, leaving the top end face of the compact 7 free, the compact 7 is forced to expand outward from the die 2. At the same time, the periphery of the compact 7 receives strong friction from the die 2. As a result, local strain occurs inside the compact 7, forming a crack 8. This crack 8 causes generation of detaching fracture.
To prevent generation of detaching fracture, in the hold-down method, application of a predetermined holding pressure PH to the compact 7 is continued until completion of the step S3 of ejecting the compact 7. This conventional hold-down method has been employed for compaction of high-hardness powder such as ceramic powder and intermetallic compound powder that are high in hardness, hard to develop plastic deformation, and poor in ductility, and has delivered sufficient effects.
However, the above conventional method has the following problem. When the pressed density of the compact is comparatively low as in the case of manufacturing an anisotropic rare earth magnet, buckling (collapse) of the compact tends to be generated. In the case of manufacturing an anisotropic rare earth magnet, powder is aligned in a magnetic field during compaction. In this case, a lubricant is added to the magnet powder, and also the compacting density is reduced by compacting the powder at a low pressure, to thereby improve the alignment of powder particles. In this case, since the compact strength is weakened, buckling may be generated in the resultant compact even with application of a comparatively small pressure.
In recent years, with expanding use of magnets, there arises the need for producing compacts having a shape elongated in the pressing direction (direction of movement of the punch). Herein, for convenience, the size of a compact measured in the pressing direction is called the xe2x80x9ccompact heightxe2x80x9d, and a typical size of the compact measured in the direction perpendicular to the pressing direction is called the xe2x80x9ccompact widthxe2x80x9d or xe2x80x9ccompact diameterxe2x80x9d. The face of the compact in contact with the upper punch is called the xe2x80x9ccompaction facexe2x80x9d, and the extent thereof is called the xe2x80x9ccompaction areaxe2x80x9d.
When the xe2x80x9ccompact widthxe2x80x9d and the xe2x80x9ccompaction areaxe2x80x9d are fixed, as the xe2x80x9ccompact heightxe2x80x9d is increased, buckling of the compact is more likely to be generated with application of pressure in the pressing direction during the step of ejecting the compact. FIG. 4A illustrates the state where pressures P1 and P2 are applied to a compact having a comparatively small compact height, and FIG. 4B illustrates the state where pressures P1 and P2 are applied to a compact having a comparatively large compact height. The trouble of buckling occurs significantly more often in the case of FIG. 4B, compared with the case of FIG. 4A.
The magnitude of the pressure in the pressing direction with which buckling of a compact is generated, that is, the buckling strength (collapse strength) decreases as the portion of the compact 7 exposed outside from the die 2 increases in the course of ejecting the compact 7 from the die 2. Therefore, in the step S3 where the compact 7 is gradually ejected while the holding pressure PH (constant pressure) is applied to the compact 7 so as to avoid generation of detaching fracture, buckling may not be generated in the initial stage of the step S3. However, in the latter stage of this step, there arises the possibility of generating buckling or collapse. In the latter stage of the ejecting step S3, a large part of the compact has been exposed, and the compact tends to be collapsed even when the applied holding pressure PH is comparatively small. The larger the compact height is, the higher the possibility of generating buckling is.
FIG. 5A shows the ranges of the compact pressure P in which detaching fracture is generated (detaching generation region) and buckling is generated (buckling generation region). In FIG. 5A, the holding pressure PH is set at a value that circumvents the detaching generation region. In the case shown in FIGS. 5A and 5B, however, since the buckling strength decreases with the progress of the ejection of the compact 7, the holding pressure PH finally enters the buckling generation region. This means that buckling is generated in the compact 7 in the latter stage of the ejecting step. If the holding pressure PH is lowered to avoid buckling, it enters the detaching generation region, causing detaching fracture this time. This problem does not arise in the case where the ratio of the xe2x80x9ccompact heightxe2x80x9d to the xe2x80x9ccompact widthxe2x80x9d or the xe2x80x9ccompaction areaxe2x80x9d is small and in the case where the compact strength is high, because in these cases, the buckling generation region shown in FIG. 5A is shifted upward.
Japanese Laid-Open Patent Publication No. 10-8102 describes control of the magnitude of the holding pressure during ejection of a compact from a die based on the height of the portion of the compact exposed from the die. However, according to an experiment by the present inventors, where compacts were ejected by the method described in the above publication, detaching fracture was generated in some cases at the start of exposure of the compact from the die. In particular, this phenomenon was often generated in the case of ejecting elongated compacts having a large ratio of the xe2x80x9ccompact heightxe2x80x9d to the xe2x80x9ccompact widthxe2x80x9d or the xe2x80x9ccompaction areaxe2x80x9d from the die.
In view of the above, it is considered necessary to control the holding pressure at the start of exposure of the compact. However, while the above publication describes control of the holding pressure after the compact has been exposed based on the height of the exposed portion of the compact, no mention is made on control of the holding pressure before the compact has been exposed.
The required holding pressure PH is markedly small compared with the pressure PC applied during compaction. However, it is very difficult to regulate the compact pressure P with high precision. Conventionally, the upper punch 3 and the die 2 are often driven with hydraulic devices. In this type of press, a conventionally adopted detects the hydraulic pressure of a compression cylinder and calculates the pressure P applied to the compact 7 based on the magnitude of the hydraulic pressure. This method is described in Japanese Laid-Open Patent Publication No. 10-152702, for example.
However, the hydraulic pressure detected in the above method varies with the mechanical resistance load received by the upper punch 3 and the die 2 when these members are driven, and thus it is difficult to precisely determine the pressure P that is being applied to the compact 7. Therefore, a new method is required to precisely detect the pressure P that is actually being applied to the compact 7 for prevention of detaching and buckling.
A primary object of the present invention is to provide a method for producing a powder compact and a method for manufacturing a magnet, where detaching fracture and buckling are reduced during ejection of a compact than in the prior art.
Another object of the present invention is to provide a powder press capable of detecting the pressure being applied to a powder compact with high precision and controlling the operation of a pressurizing member based on the detection results, and a method for driving such a powder press.
The present invention relates to a method for producing a powder compact and an apparatus for producing the same including a die having a through hole for formation of a cavity and first and second punches for compacting powder in the through hole. The method includes the steps of: filling the cavity with the powder in a state where at least an end portion of the second punch is located in the through hole of the die; producing a compact of the powder by inserting at least an end portion of the first punch in the through hole of the die and compacting the powder between the first and second punches; increasing the distance between the first and second punches while applying a pressure to the compact from the first and second punches, to thereby decrease the pressure; and starting relative movement of the die with respect to the compact after the decrease of the pressure is started and before the decrease of the pressure is halted, and completing ejection of the compact from the through hole of the die before the pressure becomes zero.
In a preferred embodiment, the relative movement of the die with respect to the compact is started when a preset time has elapsed from a time point at which the increase of the distance between the first and second punches is started.
Alternatively, the relative movement of the die with respect to the compact may be started when the pressure drops to a preset first level by increasing the distance between the first and second punches.
In a preferred embodiment, during the progress of the relative movement of the die with respect to the compact, the second punch is kept unmoved while the die is moved.
Alternatively, during the progress of the relative movement of the die with respect to the compact, the die may be kept unmoved while the second punch is moved.
In a preferred embodiment, the increase of the distance between the first and second punches is halted when the pressure drops to a preset second level by increasing the distance between the first and second punches.
Preferably, the pressure is detected based on an output of a strain sensor attached to at least one of the first and second punches.
In a preferred embodiment, the powder is magnet powder.
Preferably, the magnet powder is rare earth alloy powder having a mean particle size of 5 xcexcm or less.
Preferably, the magnet powder is produced by quenching alloy molten mass.
A lubricant is preferably added to the powder.
The method may further include the step of sintering the powder compact.
A magnetic field for alignment may be applied to the powder during the compaction of the powder between the first and second punches.
Preferably, the direction of the magnetic field for alignment in the cavity is substantially perpendicular to a pressing direction of the first and second punches against the compact.
Alternatively, the method for manufacturing a magnet of the present invention is a method for manufacturing a magnet using an apparatus including a die, a first punch, and a second punch for compacting magnet powder. The method includes the steps of: producing a compact of the magnet powder by compacting the magnet powder with the first and second punches; increasing the distance between the first and second punches to thereby decrease a pressure applied to the compact from the first and second punches; and starting relative movement of the die with respect to the compact after the decrease of the pressure is started and before the decrease of the pressure is halted, and completing ejection of the compact from the die before the pressure becomes zero.
Preferably, in the step of producing a compact of the magnet powder by compacting the magnet powder with the first and second punches, a magnetic field for alignment having a direction perpendicular to a pressing direction is generated.
In the step of producing a compact of the magnet powder by compacting the magnet powder with the first and second punches, a compact in a plate shape may be produced where a size measured in the direction parallel to the direction of the magnetic field for alignment is smaller than a size measured in any other direction.
The powder press of the present invention includes a die having a through hole for formation of a cavity and first and second punches for compacting powder in the through hole. The press executes the steps of: filling the cavity with the powder in a state where at least an end portion of the second punch is located in the through hole of the die; producing a compact of the powder by inserting at least an end portion of the first punch in the through hole of the die and compacting the powder between the first and second punches; increasing the distance between the first and second punches while applying a pressure to the compact from the first and second punches, to thereby decrease the pressure; and starting relative movement of the die with respect to the compact after the decrease of the pressure is started and before the pressure drops to a preset level, and completing ejection of the compact from the through hole of the die while the pressure is at the preset level.
In a preferred embodiment, the relative movement of the die with respect to the compact is started when a preset time has elapsed from a time point at which the increase of the distance between the first and second punches is started.
The relative movement of the die with respect to the compact may be started when the pressure drops to a preset first level by increasing the distance between the first and second punches.
In a preferred embodiment, during the progress of the relative movement of the die with respect to the compact, the second punch is kept unmoved while the die is moved.
During the progress of the relative movement of the die with respect to the compact, the die may be kept unmoved while the second punch is moved.
In a preferred embodiment, the increase of the distance between the first and second punches is halted when the pressure drops to a preset second level by increasing the distance between the first and second punches.
Preferably, the powder press further includes a strain sensor attached to at least one of the first and second punches, wherein the pressure is detected based on an output of the strain sensor.
Alternatively, the powder press of the present invention includes a die, a first punch, and a second punch for compacting powder, wherein the powder press further includes a sensor attached to at least one of the first and second punches for detecting strain of the punch, and a pressure applied to the powder from the first and second punches is determined based on an output of the sensor, for control of operations of the first and second punches.